The present invention relates to an image forming apparatus in which different toners are used to form a toner image and the thus formed toner image is fixed onto a recording sheet. More particularly, the present invention relates to a toner supply mechanism for supplying toner to a plurality of developing units in which development is conducted with different toners.
In order to reduce the size and cost of the image forming apparatus described above, the diameter of the image forming body is reduced and the size of the developing unit is also reduced. For the purpose of downsizing the developing unit, it is effective that the toner supply means is attached to the developing unit.
One of the examples of the developing units to which the toner supply means is attached is described as follows. A toner feed means for feeding toner is provided in a lower portion of a toner hopper. The thus formed toner supply means is directly connected with an upper portion of the developing unit. The above developing unit has several disadvantages, which will be described as follows. In the above developing unit, height of the toner supply means is increased. Therefore, it is difficult to downsize the image forming apparatus. Toner is supplied to the toner supply means in such a manner that toner is moved from a toner container to a toner hopper. At this time, toner tends to be spilt. In the case of an image forming apparatus in which a plurality of developing units are arranged around the image forming body, it is necessary to arrange a plurality of developing units on one side of the image forming body. In this case, the number of developing units arranged on one side of the image forming body is limited to 2 due to the limitation of the toner supply means. Further, capacities of the 2 toner supply means tend to be unbalanced.
The other of the examples of the developing units to which the toner supply means is attached is described as follows. A toner feed means of the toner supply means is composed of a toner conveyance screw arranged in a lower portion of the toner hopper, and a toner ejection port arranged on a side wall of the toner hopper on the conveyance end side. In this case, the toner ejection port is connected with the developing unit via a connection pipe. According to the above structure, degree of freedom of the position of the toner supply means with respect to the developing unit is increased by the connection pipe. Therefore, the number of developing units arranged on one side of the image forming body can be increased, and further the capacities of the toner supply means can be well-balanced. However, when toner is moved from the toner container to the toner supply means, toner tends to be spilt, and height of the toner supply means is increased, so that it is difficult to downsize the image forming apparatus.
Another toner supply means is described below. The toner supply means has a toner cartridge composed as follows. The toner cartridge includes: a toner container having a main body, one end of which is open; a cap member engaging with an outer periphery on the open end side of the main body; and a toner conveyance screw portion, wherein the gear to be driven is arranged outside an end plate of the cap member, the shaft penetrates the end plate, the rotatable screw portion extends inside along the side wall of the main body, and the end portion of the screw portion is rotatably supported by the end plate of the main body. The toner cartridge is provided with a toner ejection port formed on the cap member on the conveyance end side of the toner conveyance screw portion or formed on the side wall. The toner ejection port is connected to the developing unit via a connection pipe in such a manner that the toner cartridge can be replaced from the developing unit. In the above apparatus, the screw member is rotated.
The above toner cartridge is disadvantageous in that the number of parts of the toner cartridge is increased and the cost is raised. Therefore, the following apparatus is disclosed in Japanese Patent Publication Open to Public Inspection Nos. 2881/1991, 477/1992, 1681/1992 and 266380/1990, and also disclosed in Japanese Patent Publication Nos. 60387/1988 and 6194/1993. A toner cartridge includes: a gear to be driven which is formed on the outer circumferential surface of the barrel wall; a spiral-shaped protrusion for feeding toner which is arranged on the inner circumferential surface. That is, the toner cartridge includes: a rotational container portion for conveying toner to one end; and a stationary cap portion rotatably engaged with one end of the container so that toner can be ejected from the toner ejection port having a sliding door provided on the side wall or the end plate. The toner ejection port of the stationary cap portion is connected with the developing unit via a connection pipe so that the toner cartridge can be replaced from the developing unit. The above apparatus is advantageous as follows. When toner is moved from the toner container, no toner is spilt. Since the toner cartridge is placed sideways, height of the toner cartridge is relatively small. Therefore, it is possible to downsize the image forming apparatus. Further, it is possible to increase the number of developing apparatus arranged on one side of the image forming body, and further the capacities of the toner cartridges can be well-balanced. However, even the above apparatus has the following disadvantages: it is complicated to disconnect the connection pipe from the developing unit in the case of maintenance; further, when the connection pipe is disconnected from the developing unit, toner tends to be spilt; furthermore, it is necessary to extend the connection pipe from the toner ejection port to the developing unit.
Since a quantity of toner ejected from the toner cartridge greatly fluctuates, in order to supply toner smoothly in accordance with the toner consumption, it is necessary to store a predetermined quantity of toner between the toner cartridge and the developing unit. In order to provide a sufficient space for installing the storage portion, it is necessary to separate the toner cartridge from the developing unit in the upward and downward direction. Due to the foregoing, it becomes difficult to downsize the image forming apparatus. In the case of a color image forming apparatus, a plurality of developing units are required. In this case, the number of the connection pipes is the same as the number of the developing units. Due to the foregoing, the cost is raised. The above problems are also caused in the case of the apparatus in which the toner cartridge requiring the screw member is used.
Concerning the connection pipe, the following 2 types are known. One is a connection pipe in which a spiral conveyance screw is inserted into a flexible tube, which will be referred to as a flexible type connection pipe, and the other is a connection pipe in which a spiral screw is inserted into a solid pipe, which will be referred to as a pipe type connection pipe. The connection pipe is connected to a toner receiving port of the developing unit, or alternatively, in the case of a detachable type developing unit, the connection pipe is detachably connected to the toner receiving port by a connection means. Since it is necessary to electrically charge toner by agitating it with an agitating screw, the toner receiving port is provided in an upper portion of the agitating screw in such a manner that the toner receiving port penetrates the side wall of the developing apparatus. In this connection, in order to convey a predetermined quantity of toner by the flexible connection pipe, the inner diameter of the flexible tube is increased, so that the outer diameter is increased. When this flexible connection pipe is assembled to the image forming apparatus, it is difficult to bend the flexible pipe by a small radius of curvature, because the spiral conveyance screw inserted into the flexible connection pipe functions as a coil spring.
Accordingly, when the flexible connection pipe is bent by a small radius of curvature, the elastic deformation of the conveyance screw is increased. In general, the spiral conveyance screw is made of metal. Therefore, in order to prevent the deterioration of the bent portion of the conveyance screw, it is necessary to reduce the number of revolutions per unit hour of the spiral conveyance screw, or alteratively it is necessary to increase the radius of curvature of the bent portion. However, when the number of revolutions per unit hour of the spiral conveyance screw is reduced, a quantity of conveyed toner is lowered, and as a result, the number of image formation per unit hour is lowered. On the other hand, when the radius of curvature is reduced, the dimensions of the image forming apparatus are increased. Further, when the radius of curvature is reduced, since the flexible tube is generally made of resin, the flexible tube is damaged by the metallic conveyance screw at the bent portion. In order to assemble the pipe type connection pipe into the image forming apparatus, the connection pipe is extended in the apparatus. Therefore, end portions of solid pipe members must be connected with each other so that the conveyance direction can be changed. However, since the rotational shaft of the conveyance screw is supported by the pipe end, it is impossible to connect the pipe ends. Accordingly, the pipe connection is conducted in such a manner that a small hole, which functions as a toner current path, is formed at a position close to the pipe end so that the connection can be made through the hole. In this case, a joint portion where two pipes are joined to each other becomes a bulky node, so that the pipe extension is limited. Due to the foregoing, it is difficult to downsize the image forming apparatus. Especially, in the case of a color image forming apparatus, the connection pipes must be extended with respect to a plurality of developing units. Therefore, it is difficult to downsize the color image forming apparatus.